It seems that one option would be to put the formula = E1 + F1 in cell D1.
But maybe that is not what you want in which case you could use this routine:

Sub testing()

Dim lng As Long

For lng = 1 To 500
Range("D" & lng).Value = Range("E" & lng).Value _
+ Range("F" & lng).Value
Next lng

End Sub

Tom (is my name not part of code)